These very large aphids are commonly noticed on the bark of deciduous trees in the late summer. The aphids are grayish black, and found in dense clusters on the bark of 1-3 year old stems on infested trees. The aphids may only be noticed when they become a nuisance by wandering from infested trees or shrubs and onto sidewalks, decks and patios. The aphids leave an iodine-colored stain when crushed.

The giant bark aphid may be found on a variety of trees such as elm, sycamore, oak, maple, basswood, birch, walnut and willow. This is our largest aphid, measuring one-quarter inch in length. Its long legs make it appear even bigger. Giant bark aphid may be found throughout the growing season, but populations are largest and most easily noticed during late summer.

The giant willow aphid is found only on the stems of willow (including pussy willow and Austree). The size is approximately 3/16 of an inch with long legs that make it look even larger. Other than its size the most distinctive feature of this aphid is a large dorsal tubercle (function unknown).

Bark aphids feed on sap from inside the twigs. Heavy infestations and severe sap loss may result in stunting or more severe injury including twig dieback. However, otherwise healthy trees often sustain very large populations without any observable affects.

Controlling aphids in late summer or fall is usually not warranted. However, observable populations in early summer, especially on young or stressed plants should be controlled. Spraying before the aphids become heavily established will be much more effective. Insecticides are more effective when temperatures are warmer.

Almost all garden and landscape insecticides are appropriate for treating aphids on trees. Common synthetic insecticides such as malathion, Orthene, Sevin permethrin or esfenvalerate could be used, as well as alternative choices such as insecticidal soap or horticultural oil. Check the label of the product you are considering to make sure the tree you intend to spray is listed. Thorough coverage is essential for control. Spray the lower branches of large trees to reduce some of the annoyance wandering aphids will cause, even if the entire tree cannot be treated.

Last updated Thursday, July 22, 2010 - 16:00.

This information is valid for Iowa. It may or may not apply in your area.

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